1. Field of the Invention
The invention relates to a colour picture tube whose neck accommodates an electron gun system for emitting to a display screen, three co-planar electron beams, viz. a central beam which coincides at least substantially with the axis of the picture tube and two outer beams located on either side thereof, and including a deflection unit which is secured coaxially around the picture tube, which deflection unit comprises a system of line deflection coils which, when energized, deflects the electron beam in a first direction and a system of field deflection coils which, when energized, deflects the electron beam in a direction normal to the first direction, whilst elements of a soft magnetic material positioned symmetrically with respect to the plane through the non-deflected electron beams are provided between the systems of line and field deflection coils in the proximity of the centre of the deflection field generated by the system of field deflection coils.
2. Description of the Related Art
In colour picture tubes of the in-line type the electron gun system is adapted to generate three co-planar electron beams which converge on the display screen. The deflection unit placed around the picture tube for deflecting the electron beams is used to deflect the electron beams in one or in the other direction of their normal non-deflected straight path so that the beams impinge upon selected dots of the display screen to provide visual indications thereon. By varying the magnetic deflection fields in a suitable manner, the electron beams can be moved upwards or downwards and to the left or to the right across the (vertically placed) display screen. By simultaneously varying the intensity of the beams, a visual presentation of information or an image can be formed on the display screen. The deflection unit secured around the neck portion of the picture tube comprises two systems of deflection coils to enable the electron beams to be deflected in two directions at right angles with respect to each other. Each system comprises two coils placed on sides facing each other of the neck of the tube, the systems being displaced with respect to each other around the neck of the tube over an angle of 90.degree.. Upon energizing, the two systems of deflection coils produce orthogonal deflection fields.
The fields are essentially at right angles to the path of the non-deflected electron beams. A cylindrical core of a magnetizable material, which encloses the system of line deflection coils, if this system is of the saddle type, is generally used to concentrate the deflection fields and to increase the flux density in the deflection region.
Line deflection coils of the saddle type are coils comprising a plurality of conductors which are wound to form longitudinal first and second side packets, an arc-shaped first end segment and an arc-shaped second end segment together defining a window aperture. In such coils the rear end segments (on the side of the gun) may be flared with respect to the profile of the picture tube (the original type of saddle coil) or they may be arranged flat against the tube wall (in this type of saddle coil the rear end segment follows, as it were, the tube profile). In a hybrid deflection unit the field deflection coils are of the type which is toroidally wound on the annular core.
After mounting a deflection unit provided with field deflection coils and line deflection coils on the picture tube for which it is intended, residual errors are often found to occur during operation.
The technical possibilities of winding deflection coils are not adequate to make coils with such a winding distribution that all requirements relating to convergence and raster are simultaneously satisfied. In a known deflection unit a number of soft magnetic elements are provided which influence the magnetic field of the system of coils providing vertical deflection and thereby reduce residual convergence errors in the corners of the display screen. In contrast thereto, geometrical (raster) errors are corrected, if necessary, by using permanent magnets which are mounted on the outside of the coils against the side of the coil holder which is to face the display screen.
Number, shape and size of the conversions correction elements, as well as their position with respect to the system of coils for vertical deflection must meet stringent requirements in any type of deflection unit. For units of the same type these requirements are, however, equal. The soft magnetic elements therefore do not serve to eliminate individual differences in the properties of the units of one type, but they are to correct residual errors relating to convergence in the design of a deflection unit type.
Astigmatism correction and coma correction elements have been known so far as correction elements (or in other words: field shapers). The astigmatism correction element is provided in the proximity of the centre of the deflection field generated by the system of field deflection coils and comprises soft magnetic elements which are positioned symmetrically with respect to the plane through the non-deflected electron beams. It is used both in combination with field deflection coils of the saddle type and of the toroidal type wound on the annular core. The coma correction element is only used in combination with field deflection coils of the saddle type. The coma correction element comprises soft magnetic elements which are positioned symmetrically with respect to the plane through the tube axis which is at right angles to the plane through the non-deflected electron beams, and is provided at the entrance side of the deflection field generated by the system of field deflection coils where there has been hardly any or no pre-deflection. In colour picture tubes which comprise a deflection unit with a system of field deflection coils of the toroidally wound type coma correction is generally effected within the tube. To this end the coma correction elements are provided on the electron gun system, for example, one element for each of the two outer beams.
Despite the use of astigmatism correction elements in the system of field deflection coils, astigmatism errors of higher order components of the deflection field are still found to occur in the present-day colour picture tubes.